Many people seem to justify a switch to Ubuntu LTS with the argument of
5-year security support. This support only applies for packages in
main. A common example is nginx which is in universe. Packages in
universe are just unsupported. They may or may not get any security
support. If you need to advocate for Debian vs Ubuntu, I think this is a
strong argument.

The fact that Debian only supports stable releases for about 1 year after the release of the next version does not exactly help make a case against Ubuntu, even now that non-LTS get only 12 months (instead of the former 18 months) of support. Maybe longer support terms for stable versions of Debian would help (3 years?).

Also, many advanced users of Ubuntu end up contacting the Debian packager for updates, fixes, backports, etc Even through Launchpad directly (the Debian maintainer is shown as the package maintainer and receives e-mail automatically)